A well-known problem with currently manufactured organic light emitting diodes (OLEDs) is that the efficiency of the process by which they convert electrical energy into light is greatly compromised by loses of light inside the devices. In these devices the molecules of organic light emitting material in a light emitting layer emit light more or less uniformly in all directions. Light emitted at off vertical angles is trapped inside the device and eventually absorbed through the same process of total internal reflection utilised in light pipes and optical fibres. In general over 50% of light emitted is lost in this way. If this light could either be recovered or its emission suppressed then the efficiency of OLEDs would be greatly improved and the energy efficiency of OLEDs could be made to be much higher than that of currently known light sources and electronic displays.
One potential approach to limiting the amount of light emitted in the plane of the OLED devices and thus increasing their efficiency is to utilise liquid crystalline OLED emitter materials. These materials have long, rod-like molecular cores defining a long axis along the core length. As a result of their shape they are highly dichroic. That is to say, in each molecule most of the generated light is emitted along axes that are perpendicular to the long axis of the molecule, with very little light being emitted in the direction parallel to the long axis. Thus if a liquid crystalline emitter material is deposited to form the OLED light emitting layer with its molecular cores parallel to the plane of the device i.e. in-plane (as is most often the case) the amount of light emitted in-plane will be decreased as compared to the amount emitted perpendicularly to this plane.
Unfortunately liquid crystalline OLED materials exhibit a countervailing effect. The long molecular axes of these materials leads to their having an unusually high refractive index for light polarised with its axis of polarisation parallel to the molecular long axis. This increases the effective in-plane refractive index of the material leading to increased internal reflection and more light capture in the device plane, reducing efficiency.
Hitherto no suitable electroluminescent device structure has been found or method that effectively overcomes this problem associated with OLED structures and in particular structures based on liquid crystalline OLED materials.